1. Field of the Invention
This invention relates to a power conversion system, and more particularly to a power conversion system composed of voltage source type self-commutated converters in a multiplexed configuration and a control system of these voltage source type self-commutated converters when these voltage source type self-commutated converters are applied to such systems as DC transmission systems, fuel cells, battery energy storage systems, and reactive power compensation systems.
2. Description of the Related Art
A power conversion system composed of conventional voltage source type self-commutated converters in a multiplexed configuration and a control system of these voltage source type self-commutated converters will be described hereinafter with reference to FIG. 14 and FIG. 15. In FIG. 14, 1 is an AC system, 2 is a potential transformer to measure an AC system voltage of AC system 1, 3 is a current transformer, and 4A, 4B are converter transformers to connect voltage source typo self-commutated converters 5A, 5B to AC system 1. 6 is a DC voltage source composed of DC capacitor, cell, etc., 7 is a DC voltage detector, and 8 is an active/reactive power detector to detect active and reactive powers that are output from converters 5A, 5B by inputting detected values of potential transformer 2 and current transformer 3. 9 is a DC voltage controller to input a difference between a DC voltage reference 51 and a DC voltage detected value 52 detected by DC voltage detector 7 and to control the DC voltage so as to make the difference zero. 10 is a reactive power controller to input a difference between a reactive power reference 53 and a reactive power detected value 54 detected by active/reactive power detector 8 and to control the reactive power so as to make the deference zero. 11 is an AC current controller to control the AC current to a reference value by inputting an output of DC voltage controller 9, an output of reactive power controller 10, an AC current detected value 55 detected by current transformer 3 and an AC voltage detected value 56 detected by potential transformer 2. 12 is a pulse width modulation circuit to decide pulse patterns of self-turn-off devices composing each of self-commutated converters 5A, 5B according to the output of AC current controller 11.
In FIG. 15, 13A-13L are self-turn-off devices such as gate turn-off thyristors (hereinafter referred to as GTO) and 14-14L are diodes. Furthermore, 1A, 1B and 1C designate A-phase, B-phase and C-phase of AC system 1, respectively. The principle of control of active/reactive power of voltage source type self-commutated converters 5A, 5B connected to AC system 1 is disclosed in a publication titled "Semiconductor Power Conversion Circuit" (published from The Institute of Electrical Engineers of Japan), P.215-220, etc. and the detailed description will be therefore omitted here. In addition, the principle and realizing a method of a constant current control circuit are disclosed in Japanese Patent Publication (Kokai) No. Hei 1-77110, and therefore, the detailed description will be omitted here.
In FIG. 15, the AC system side windings of converter transformers 4A, 4B are connected in series, the DC side winding thereof are connected to each of converters 5A, 5B individually, and the DC outputs of voltage source type self-commutated converters 5A, 5B are connected in parallel with each other. In this configuration, as for the AC system output voltage, the outputs of converters 5A and 5B are added through the AC windings of converter transformers 4A, 4B and higher harmonics thereof are negated. In addition, as the AC windings of converter transformers 4A, 4B are connected in series, the current values of converters 5A, 5B become the same. Further, as the DC sides of converters 5A, 5B are connected in parallel with each other, the DC voltage of converters 5A, 5B become the same. In the configuration shown in FIG. 14, higher harmonics can be decreased in the AC system output voltage by converters 5A, 5B, and in addition, no unbalanced current and voltage will be generated between converters 5A, 5B.
The multiplexed configuration of conventional voltage source type self-commutated converters shown in FIG. 14 has the following problem when considering the application of it to, such as, a DC transmission system to transmit DC power over a long distance. That is, as DC outputs of the converters are connected in parallel, if the number of converters is increased for attaining the large capacity, DC current will increase accordingly. In case of the DC transmission system, a DC line is long and the resistance of a DC transmission line is large. If DC current increases, the power loss by the resistance of the DC transmission line also increases in proportion to a square of DC current, and the efficiency of the system will drop. Therefore, when applying the conventional power conversion system to, such as, the DC transmission system, in case of increasing the system capacity, it is preferred to increase the DC voltage rather than increasing the DC current from the viewpoint of reducing the loss.
It can be also considered, as one method to increase the DC voltage, to increase rated DC voltage of the converters and thereby make rated DC current small. However, in case of self-commutated converters with a large capacity, as it is not possible to increase switching frequency of the converter from the viewpoint of reducing the switching loss, the multiplexing of converters becomes indispensable for reducing higher harmonics.
As described above, instead of the conventional multiplexed configuration of converters to connect the DC sides of converters in parallel, a power conversion system with the multiplexed configuration of converters and its control system which does not increase DC current is demanded.